Influence of humidity on granular packings with moving walls

A significant dependence on the relative humidity H for the apparent mass (M-app) measured at the bottom of a granular packing inside a vertical tube in relative motion is demonstrated experimentally. While the predictions of Janssen's model are verified for all values of H investigated (25% less than or equal to H less than or equal to 80%), M-app increases with tune towards a limiting value at high relative humidities (H greater than or equal to 60%), but remains constant at lower ones (H = 25%). The corresponding Janssen length lambda is nearly independent of the tube velocity for H greater than or equal to 60% but decreases markedly for H = 25%. Other differences are observed on the motion of individual beads in the packing: for H = 25%, they are almost motionless, while the mean particle fraction of the packing remains constant; for H greater than or equal to 60% the bead motion is much more significant and the mean particle fraction decreases. The dependence of these results on the bead diameter and their interpretation in terms of the influence of capillary forces are discussed.